As @giraff mentioned in the comments, this is a question of context and purpose. If your countdown shows the time till the next Olympics, then of course you'll be using days (also months and years if necessary). Your goal is just to display the time in the clearest way, so it's best to use the same units the user usually uses.
But in a social buying context, your goal is different - you're trying to convey a sense of urgency and have the user feel like time is running out, and he should make the purchase ASAP. In this context you don't want him thinking in terms of days, so you use hours.
Another related point - showing days-hours-minutes-seconds feels weird. When you display days, you usually don't need the seconds resolution, and vice versa. And here you definitely need seconds, because you want the user to see the time elapsing before his eyes, it drives him to action. You can't get this effect with just minutes.